Don't worry about the market or audience at this point. Write what feels most natural and good for you. When you write with a target--and not your heart--in mind, it shows. (I know, I know...that sounds very Zen like, but a lot of writers say this, and I can verify it with personal experience).
As for the character/idea problem, start writing the idea and see if the characters can come together as you do it. It might be useful to do exercises with them, writing up bios or trait sheets. There are a lot of great chracter trait books out there, and while it may seem formulaic, starting with those can actually spark some ideas. You may then find the characters starting to drive the story as you write it, the more you start to think about them.